1. Field of the Invention
This invention relates to a zoom lens capable being used with a small-sized machine having an imaging function, particularly a digital still camera, a mobile telephone with a camera, a personal digital assistance (PDA), etc.
2. Description of Related Art
In recent years, further miniaturization of an imaging apparatus such as a digital still camera as the whole apparatus has been demanded with advance of miniaturization of an imaging device of a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), etc. Accordingly, slimming down of an imaging lens, particularly a zoom lens by shortening the entire length, etc. Hitherto, for a zoom lens having a variable power ratio of about ×3 of zoom lenses used with a digital still camera, etc., a zoom lens made up of three groups has been advantageous for shortening the entire length and has been widely used (Refer to JP-A-10-293253). Each lens group of the zoom lens is movable and although a length to some extent is required when used, the spacing between the groups is narrowed as much as possible when non-used, whereby slimming down is realized. In recent years, however, strong durability against shock of drop, etc., has been demanded for a digital still camera, etc., and the zoom lens made up of three groups described above has a structure wherein a lens barrel for supporting the moving lens groups when used pops up from the imaging apparatus main body and thus there is a limit for increasing the durability.
Then, as a configuration of a zoom lens to provide compatibility between slimming down and strong durability, in a four-group configuration of first positive, second negative, third positive and fourth positive groups, a right-angle prism is placed in the first lens group for bending the optical path approximately 90° at midpoint to form a refractive optical system, thereby shortening the length of the optical system in the thickness direction thereof (Refer to JP-A-8-248318, JP-A-2000-131610 and JP-A-2003-202500). In the configuration, the first lens group is fixed when zoomed and focused and thus a structure wherein the entire lens barrel can be housed in the imaging apparatus main body can be produced, making it possible to increase the durability as compared with the zoom lens made up of three groups described above.
However, the zoom lens described in JP-A-8-248318 is inappropriate for slimming down because the first lens group is large and the overall number of lenses is also large although the zoom lens has a large zoom ratio and a bright F value. The zoom lens described in JP-A-2000-131610 has a small number of lenses and a large angle of field at a wide angle end, but the first lens group is large and the right-angle prism has a low refractive index and is hard to totally reflect and thus reflecting coat becomes necessary on the reflection face of the right-angle prism, resulting in lowering of transmittance in the whole system and an increase in the cost. The zoom lens described in JP-A-2003-202500 involves various examples, which contain some examples of compact zoom lenses with a small number of lenses, but high-refractive index and high-dispersion material is used for the right-angle prism in every example and the transmittance on the short wavelength side of a visible light beam lowers. That is, in JP-A-2003-202500, a right-angle prism of a material with the refractive index at the d-line exceeding 1.8 and the Abbe number less than 30 is adopted as an example; under present circumstances, however, such a high-refractive index and high-dispersion material having sufficiently high transmittance does not exist and is not practical.